who
people and sometimes pet animals
defining and non-defining
which
animals and things
defining and non-defining; clause referring to a whole sentence
people, animals and things; informal
defining only
possessive meaning;
for people and animals usually; sometimes for things in formal situations
defining and non-defining
whom
people in formal styles or in writing; often with a preposition; rarely in conversation; used instead of who if who is the object
defining and non-defining
no relative pronoun
when the relative pronoun defines the object of the clause
defining only
(In the examples, the relative pronoun is in brackets to show where it is not essential; the person or thing being referred to is underlined.)